1. Field of the Invention
The present invention relates to a method for controlling a solenoid valve, e.g., a proportional solenoid valve, which is used for controlling the pressure in a hydraulic system, such as ABS (antilock braking system) and/or ESP (anti-slip regulation) in a motor vehicle.
2. Description of Related Art
In implementation of vehicle dynamics control systems such as ABS, ESP, ASR (electronic stability program) and the like, a proportional solenoid valve is used for a targeted build-up of pressure or reduction in pressure in the hydraulic system. The hydraulic system may include, for example, a hydraulic brake circuit, a hydraulically operable clutch in an automatic transmission, hydraulic actuators for influencing driving dynamics or the like. Known controllers of a proportional solenoid valve for ABS/ESP systems assume either a strict steady-state behavior of the proportional solenoid valve (dp method), a strict switching behavior (quasi-switching method) or a linear valve behavior (LMV method). The choice of the method to be used depends on the stipulated pressure gradient. In estimating the pressure in the brake cylinder, in the known approaches it is always assumed that the setpoint pressure is reached accurately at the end of the actuation of the proportional solenoid valve, inasmuch as this is implementable within the scope of physical limits.
Due to the variety of different methods, unambiguous switchover conditions between the different methods must be defined. Therefore, valve triggering, pressure estimation and the calibration procedure are very complex. Important influencing parameters, such as the inductance of the coil of the proportional solenoid valve, the nonlinear relationship between the valve flow rate and the coil current, or the pressure difference are not taken into account in any of the aforementioned known triggering methods. Likewise, the dynamic response and the nonlinearity of the proportional solenoid valve are not taken into account in the pressure estimate, which results in errors in estimation of the brake pressure and must be taken into account heuristically by introducing additional terms.